(NECN: John Moroney, Boston) - A U.S. Coast Guard helicopter spent much of the day in the air over Boston Harbor and Castle Island, looking for a 41-year-old man who fell off a ship Saturday night during a country-music themed cruise.

The Provincetown II is owned and operated by the Bay State Cruise Company, which say the incident happened around 10:35 p.m.. The still unidentified man was on board with his girlfriend.

The cruise company says he was engaged in horseplay when he jumped up on a third deck railing and tumbled into the water. People who watched the search from Castle Island said the unfortunate accident should serve as a reminder for all of us.

Boston Police are helping the Coast Guard. Once the fog lifted in Boston Harbor, it made the search for the missing passenger much easier.

State police are also helping, along with a number of other agencies. The crew of the Provincetown II said it didn't see the passenger's body surface. There were more than 350 people on board and the ship was traveling at 5 mph.

Conditions for the search were good. The seas in Boston Harbor were calm, while the water temperature was 65 degrees.